Are people that uninterested in Austria-Hungary?
There is some interest, at least, but you did pose a difficult question. Personally, I just don't see any particular difference occuring, except heaping more tragedy on top of Franz Josef.
Though, if he does live to 1928 (to be 98 years old), it would, possibly, change the events in Hungary. The attempted coup to restore Karl was a failure, but Franz Josef's reputation might change the outcome. It's unlikely, though. Horty threw in against the coup for the simple reason that no country in Hungary's vicinity would tolerate a Habsburg restoration (seeing it as a threat to them). If it happens anyway (and provided Hungary doesn't get invaded by every one of her neighbours, again), the country would remain politically isolated and, provided the Nazis come to power, you're looking at one less ally for Germany in WWII ... since Hitler despised the Habsburgs (and since, at this point, Karl would be in charge, it would very much be mutual).
Either way, I don't see either Franz Josef or Charles living longer as truly affecting anything long-term.